Finding out how underreplicated a broker is

In the output of kafka-topics.sh -describe, it gives the ISR. What I want to find out is why a particular broker is not in ISR, how far behind it is, and when it will get back into the ISR. I can't seem to find this information in the logs, cmdline output, or zookeeper. How can I find out how far behind the ISR a particular broker is?

SimonC